To assess the impact of operative relief of left ventricular outflow obstruction on myocardial perfusion abnormalities in obstructive hypertrophic cardiomyopathy, 20 symptomatic patients who underwent a septal myectomy or mitral valve replacement were studied with assessment of myocardial perfusion during exercise by thallium-201 emission computed tomography prior to and 6 months following surgery. Prior to surgery, 18 patients had myocardial perfusion defects during exercise, indicative of myocardial ischemia in 15 patients. Following surgery, with relief of left ventricular outflow obstruction, 11 patients had complete normalization of their exercise thallium studies, and 5 additional patients demonstrated improvement in the magnitude and distribution of thallium perfusion defects. However, 4 patients acquired new fixed thallium defects as a consequence of surgery and experienced a greater fall in left ventricular ejection fraction than the remaining patients. Thus, operative relief of left ventricular outflow obstruction results in normalization or improvement in myocardial perfusion in the majority of patients with reversible and fixed perfusion defects by thallium scintigraphy suggesting relief of stress - induced myocardial ischemia. However, surgery may result in myocardial injury and scarring with consequent fall in left ventricular ejection fraction in some patients.